This invention relates to a method of pumping up electric power generation and an apparatus therefor employing a single-speed reversible pump-turbine in which the direction of a water current is reversed when the operation mode is switched between generating or turbine operation and pumping or pump operation.
Pumped storage power plants, which have conventionally been constructed and in use, are confronted by a keen demand for high-efficiency operation of their pump-turbines from an economical point of view, the pump-turbine and the method of operation thereof requiring review. Now there will be given an outline of the arrangements of such conventional pumped storage power plants and the method for operating the same.
A reversible pump-turbine generally used in one such pumped storage power plant is operated as a pump (hereinafter referred to as pump operation) and also as a turbine (hereinafter referred to as turbine operation) by reversing the rotating direction of a runner and a rotating part coupled thereto. Let it be supposed that the rotating speed of the runner, pump total head and efficiency for the pump operation are Np (rpm), Hp (m) and .eta.p, respectively, and that the rotating speed of the runner, turbine net head and efficiency for the turbine operation are Nt (rpm), Ht (m) and .eta.t, respectively. FIG. 1 shows respective efficiency curves or envelope curves .eta.p and .eta.t of unit speeds per one meter head Np/.sqroot.Hp and Nt/.sqroot.Ht for the pump and turbine operations of the pump-turbine, obtained by varying the guide vane opening of the pump-turbine. As may be seen from the drawing of FIG. 1, the pump and turbine operations show different unit rotating speeds for the maximum efficiency; the maximum efficiency (point P) is obtained where Hp =Hpo, that is, where the unit speed per one meter head is Np/.sqroot.Hpo in the pump operation, whereas in the turbine operation the maximum efficiency (point Q) is attained where Ht =Hto, that is, where the unit speed per one meter head is Nt/.sqroot.Hto. As long as the reversible pump-turbine is used, it is hydraulically inevitable that the maximum-efficiency unit rotating speeds for those two operation modes will be different from each other. Although a great deal of effort has been made to minimize such difference, the ordinary ratio between both unit speeds per one meter head is at present approximately 1.1. That is, ##EQU1## Moreover, it may be seen from FIG. 1 that the efficiency of the pump-turbine is considerably low in both cases where the pump-turbine is operated as a pump at a unit speed per one meter head of Nt/.sqroot.Hto and where it is operated as a turbine at a speed of Np/.sqroot.Hpo.
Meanwhile, the direction of a water current flowing through the reversible pump-turbine in the pump operation is opposite to that in the turbine operation. Therefore, if the pump-turbine is operated with a give static head Hst (water level difference between upper and lower reservoirs while the pump-turbine is not in operation), the turbine net head Ht for the turbine operation and the pump total head Hp for the pump operation are calculated as follows: EQU Ht=Hst-.DELTA.Ht, EQU Hp=Hst+.DELTA.Hp. (2)
Here .DELTA.Ht is a head loss of a waterway in the turbine operation, while .DELTA.Hp is a head loss of the waterway in the pump operation.
As is evident from eq. (2), the turbine net head Ht, with the fixed static head, is smaller than the pump total head Hp by Hp -Ht or .DELTA.Hp +.DELTA.Ht. The value of .DELTA.Hp +.DELTA.Ht varies with the waterway conditions of pumped storage power plants. In actual pumped storage power plants where the static head Hst varies during operation of the pump-turbine, however, the ratio of the average Hp of the pump total head Hp to the average of Ht of the turbine net head Ht on the pump-turbine is usually about 1.05. That is, EQU Hp/Ht=1.05 (3)
In most pumped storage power plants, therefore, the reversible pump-turbine may function with the maximum efficiency both in pump and turbine operations for such average head condition only if Np and Nt are so determined as to have the following relationship obtained by substituting the condition of eq. (3) into eq. (1). EQU Np/Nt.congruent.1.13. (4)
Namely, in this case, it is necessary to employ a two-speed reversible pump-turbine of which rotating speed for turbine operation is lower than that for pump operation.
In order to actualize the aforesaid two-speed operation system, there was proposed such a method that the number of poles of a synchronous generator-motor coupled to a runner is changed, though it failed to be widely used because of the generator-motor's complexity.
Thus, the fact is that mainly used is a system such that the reversible pump-turbine is operated at a single speed, with the efficiency of either pump or turbine operation being enevitably low.